The present invention relates to traction modifying devices, and more particularly, to such devices of the type which are capable of operating as limited slip differentials during at least part of their operating cycle. The present invention is preferably also capable of operating as a locking differential during part of its operating cycle, and when fully disengaged is capable of operating as an open differential, such that the differential of the present invention is preferably modulatable between open, limited slip and locking modes of operation.
Limited slip and locking differentials of the type to which the present invention relates typically include a gear case defining a gear chamber and disposed therein, a differential gear set including at least one input pinion gear and a pair of output side gears. A clutch pack is typically disposed between one of the side gears and an adjacent surface of the gear case, such that the clutch pack is operable to retard, or even prevent, rotation between the side gears and the gear case. In a typical locking differential, there is a cam means disposed between the clutch pack and its adjacent side gear, the cam means being operable to engage the clutch pack upon relative rotation of the cam members and exert sufficient force on the clutch pack to lock the differential. As used herein, the term xe2x80x9clockxe2x80x9d refers to the ability to limit differentiating action of the gear set to the extent that no substantial differentiation occurs.
In many limited slip and locking differentials, some sort of actuating mechanism is provided to actuate or bias the clutch pack to its engaged condition. One of the current trends in the field of vehicle traction modifiers involves the desire to be able to actuate a clutch pack in response to an external signal (for example, an electrical signal generated by a vehicle microprocessor), rather than in response to the sensing of a predetermined speed differential. Actuating the clutch pack of a locking differential has been done in the prior art by a mechanical device, for example, by means of a flyweight mechanism. Such locking differentials of the xe2x80x9cmechanicalxe2x80x9d type are sold commercially by the assignee of the present invention. Mechanical locking differentials of this type are illustrated and described in U.S. Pat. No. 4,389,909, assigned to the assignee of the present invention, and incorporated herein by reference.
As was mentioned previously, there is an increasing number of vehicle applications in which it is desirable to be able to begin to engage the clutch pack in response to an external signal, the signal preferably being an electrical input signal, in which case the typical actuation mechanism would comprise an electromagnetic coil. An example of the type of electromagnetic actuator which could be utilized, and receive an external electrical signal, is shown in U.S. Pat. No. 5,911,643, assigned to the assignee of the present invention and incorporated herein by reference. In the above-cited ""643 patent, the electromagnetic coil is generally annular and is disposed in a face-to-face relationship with an adjacent ramp plate of a ball ramp actuator, such that energizing the coil initiates the ramp up of the ball ramp actuator.
In designing and developing limited slip and locking type traction modifiers, there are a number of different design criteria, but for purposes of the present invention, there are three which are of particular interest. The first is to be able to activate and deactivate the traction modifier very quickly. The second is being able to achieve the required axial force on the clutch pack to generate the desired xe2x80x9cbias torquexe2x80x9d, i.e., the torque which is transferred from a spinning wheel (having poor traction) to the stationary wheel (having good traction). The third design criteria of interest in regard to the present invention is to be able to disengage the clutch pack, and the associated engagement and actuation mechanism, after the poor traction situation has occurred, when it is again acceptable for the differential to operate in an xe2x80x9copen differentialxe2x80x9d mode, i.e., with little or no traction modification occurring, as opposed to the traction modifier remaining xe2x80x9cstuckxe2x80x9d in the locked mode as is known in regard to prior art devices.
Unfortunately, the kinds of modifications which would typically be made in the actuation mechanism of the differential, in order to increase the load applied to the clutch pack, and therefore, increase the bias torque, make it more difficult to disengage the clutch pack and the associated actuation mechanism. Conversely, the kinds of modifications that would normally be made to facilitate the disengagement of the clutch pack would typically result in a decreased load being applied to the clutch pack, and therefore, a decreased bias torque.
Accordingly, it is an object of the present invention to provide an improved traction modifying differential in which the clutch pack may be actuated in response to an external electrical signal, wherein the differential overcomes the problems associated with the prior art devices.
More specifically, it is an object of the present invention to provide an improved traction modifying differential which is capable of applying a greater load to the clutch pack, but which still is capable of being easily disengaged.
It is another object of the present invention to provide an improved traction modifying differential which achieves the above-stated objects and, at the same time, is capable of being activated and deactivated more rapidly than prior art devices.
The above and other objects of the invention are accomplished by the provision of an improved differential gear mechanism of the type including a gear case and defining an axis of rotation and a gear chamber. A differential gear set is disposed in the gear chamber and includes at least one input gear and first and second output gears. A retarding mechanism for retarding differentiating action of the differential gear set includes a clutch pack disposed adjacent the second output gear, and operable between an engaged condition, effective to retard relative rotation between the output gears and the gear case, and a disengaged condition. The retarding mechanism includes a first ball ramp actuator disposed adjacent the first output gear and further includes an electromagnetic actuator disposed adjacent the first ball ramp actuator and operable, in response to an electrical input signal, to cause ramping action of the first ball ramp actuator. A plurality of elongated engagement members have upstream end portions generally associated with the first ball ramp actuator and are operable to move axially toward the second output gear in response to the ramping action of the first ball ramp actuator. The elongated engagement members have downstream end portions operably associated with a clutch pack loading mechanism and operable, in response to the axial movement of the engagement members to apply a load to the clutch pack tending to change the clutch pack from the disengaged condition to the engaged condition.
The improved differential gear mechanism is characterized by the clutch pack loading mechanism comprising a second ball ramp actuator disposed axially between the second output gear and the clutch pack, the second ball ramp actuator including a rotatable ramp plate. The clutch pack loading mechanism further comprises a cone clutch assembly comprising an input member, operably associated with the downstream end portions of the engagement members. The cone clutch assembly defines a first cone clutch surface and the rotatable ramp plate defines a mating, second cone clutch surface, the first and second cone clutch surfaces being moved toward engagement in response to the axial movement of the engagement members, to initiate ramping action of the second ball ramp actuator.